Notes on the technique of the otter field survey

Acta Theriologica 41 (2): 199-204, 1996.

FRAGMENTA THERlOLOGlCA

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Notes on the technique of the otter field survey

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Jerzy ROMANOWSKI, Marcin BRZEZINSKI and Jakub P. CYGAN

Romanowski J., Brzezinski M. and Cygan J. P. 1996. Notes on the technique of the otter field survey. Acta Theriologica 41: 199-204. Data (2083 sites visited) collected during the otter Lutra lutra (Linnaeus, 1758) survey of Poland in 1991-1994 were used to evaluate some of the factors affecting the efficiency of the technique. The success in detecting otter signs was found to be significantly affected by the presence of "spraintable" bridges and other potential sprainting sites on the banks. Decreased efficiency of the technique was noted in specific habitats, eg large rivers, canals surrounded by open fields or meadows, and any aquatic habitats with few potential sprainting sites on the banks. It is suggested that modifications: spot checks at additional bridges and extended searches of river banks, could be used to improve the reliability of the survey in areas with low numbers of otter signs. Institute of Ecology, Polish Academy of Sciences, 05-092 Lomianki, Poland (JR, JPC); Department of Ecology, University of Warsaw, Krakowskie Przedmiescie 26/28, 00-927 Warszawa, Poland (MB) Key words: Lutra lutra, field survey, sprainting sites

Introduction The standard field survey method, based on the search for otter Lutra lutra (Linnaeus, 1758) signs, has been succesfully used in many European countries (for references see Macdonald and Mason 1994). The survey is usually carried out based on the system of 10 x 10 km square grid, where around 6 sites are visited in each square (Lenton et al. 1980). The basic assumption of this method is that the 600 m-long walk along the waterside is enough to find spraints (excrements) or tracks of otters inhabiting an area (Macdonald 1983). Finding otter signs indicates the presence of these animals (positive sites), whereas the absence of signs at a series of sites (negative sites) enables to state, with high probability, that otters do not occur in the area, eg along one river. The national otter survey undertaken in Poland in 1991-1994 was based on the modified standard method, with one site visited in each 10 x 10 km square (Brzezinski et al. 1996). Numerous data (2083 sites visited) collected in various habitats enabled us to critically assess the method used and evaluate some of the factors affecting the efficiency of the search. [199]

200

J. Romanowski et al.

Otter signs under bridges At most sites we began the search for otter signs under a bridge. To supplement the main survey we performed spot checks under additional bridges (see Lenton et al. 1980). In most of those 10 x 10 km squares which were negative after the initial search, 1-3 spot checks were conducted. There were two reasons for choosing the bridges: the first one was the ease of approaching the river, the second - the longevity of otter signs under the bridge cover. Otter spraints remain on the banks from several days to several weeks but in sheltered places they can exist as long as 12 months (Jenkins and Burrows 1980, Macdonald and Mason 1988). During our studies otter signs were found under bridges in 56.9% of all posi­ 70 tive sites. It should be mentioned, however, 60 that not all bridges were suitable for the survey. Bridges which had boulders, natural 50 ~ banks, wooden or concrete ledges underneath were frequently used by otters as sprainting 1) 40 c places. Bridges which had none ofthese fea­ (]) 6­ 30 tures did not offer good sites for sprainting. ~ u.. Bridges on large rivers provided places for 20 marking which did not differ from the 10 other sections of the bank (ie had similiar vegetation cover) and were rarely used by o ~ otters. On rivers up to 5 m wide 70% of 5 10 15 20 25 30 40 50 60 500 otter signs were found under the bridges, River width (m) whereas on rivers wider than 60 m none were found (Fig. 1). Fig. 1. Percentage of otter signs found under The abundance of proper ("spraintable") bridges in relation to varying river width. bridges increases the probability of detecting otter presence in a 10 x 10 km square. In fact bridges seem to be the only places to find otter spraints or tracks in habitats such as irrigation canals, drains, and chanalized rivers, or habitats where low banks were covered with dense riparian vegetation. Also in the case of lakes otter signs were easiest to fmd under small bridges on little tributaries or canals connecting two lakes.

Type of the river bank The ease of detection of otter signs, measured by the distance of the search, depends on the density of these signs on the river banks. Some authors discussed whether spraint densities could be related to habitat quality, otter numbers or activity (Macdonald and Mason 1983, Jefferies 1986, Kruuk et al. 1986, Kruuk

and Conroy 1987, ~ presence of potential however, not enough defecate at any place prominent sites. Also is much lower than JCO, habitats, such as mo this case boulders and any part oftheir ran?\? sites may result in t:b2 In this case results '~, conspicuous sites In our field study the banks surveyed rn , and "++" - > 10 sites significantly more (17.6%) (p < 0.001. G otter tracks or SPTa.:L:;::~ ie presence of expo5lEdj survey in England h~,' a low frequency of 8'" '

Previously publishe always found within Macdonald 1986. 1913, 1991). In the national the first 200 m (incl Our data showed. hu (93% signs found (p < 0.025, G = 5.0S.. G on small rivers; Fig. 1 Iii distance of the searen the search in 92 C3-~ 30 m wide) riversera small sample size eM G = 2.42, G-test~. Above observatitms , on large rivers. Dnr Mickevicius (1995 ,. compared to the STIP ":

Technique of the otter field survey

,'"' rodge. To supplement ",i bridges (see Lenton '1iiiS.""e negative after the -ere two reasons for :: me river, the second ~ts remain on the ~ pater level, and ·..:,ne:red.

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previous sug­ _.:;cd ...-ith this method .• e sites and those .oJ 'd not change much o:r.·""'p~e. in the Mazury 1: 42:.. 2% of positive - 'j.,..,..., me first 200 m. I. ~'" ,expressed by the '